For both diagnostic and therapeutic it is important to determine lung volume, its subdivisions, and divergence of volumes from normal. The diagram in FIG. 1 shows the changes in volume (on the vertical axis) against time (on the horizontal axis) as an individual breath, initially tidal volumes, and then inspires and expires maximally. As is clearly illustrated, the Total Lung Capacity (TLC) comprises a Vital Capacity (VC) and a Residual Volume (RV). The Vital Capacity could be measured with spirometry, and comprises a Tidal Volume (TV), an Expiratory Reserve Volume (ERV) and an Inspiratory Reserve Volume (IRV). The Expiratory Reserve Volume in combination with the Residual Volume is denominated as Functional Residual Capacity (FRC).
Perhaps the volume that has received most attention is the Functional Residual Capacity (FRC), the volume of gas in the lung after a normal expiration. The FRC is determined by the balance between the inward elastic recoil of the lungs and the outward recoil of the thoracic cage. FRC decreases with paralysis and anaesthesia. Other factors influencing the FRC include:
Body size
Gender
Posture
Lung pathology.
Accordingly, it is in many cases important to estimate and/or monitor the Functional Residual Capacity (FRC). Even though this is important for patients with spontaneous breathing, it is most important in ventilator treated patients as FRC is involved in gas exchange and ventilation/perfusion ratio and may be used as an indicator of atelectases. In intubated patients FRC may also be manipulated by Positive End Expiratory Pressure (PEEP) level in order to avoid cyclic collapse of lung as part of a protective ventilatory strategy. The PEEP should be held at a level high enough to avoid cyclic collapse of the lung, but low enough to avoid over-distension of the lung.
However, conventional spirometry cannot provide any of the volumes that include RV, the residual volume left in the lung after maximal expiration. Other methods are therefore required to find FRC. Methods used in the intensive care unit (ICU) for FRC monitoring are based on dilution of a bolus of low solubility gas or long term sampling of expiratory gas. However, these methods require expensive mass spectroscopy or calibrated infrared and acoustic detectors and are therefore of limited clinical use. Other methods are based on a step-change in nitrogen concentration, such as a change in inspired oxygen concentration by more than 30% nifo, which may not be feasible in a patient with accute respiration failure. Such known methods are e.g. disclosed and discussed in:    Larsson A, Linnarsson D, Jonmarker C, Jonson B,    Larsson H, Werner O (1987): “Measurement of Lung Volume by Sulfur Hexafluoride Washout during Spontaneous and Controlled Ventilation: Further Development of a Method”, Anesthesiology 67:543-550    East T D, Wortelboer P J M, van Ark E, Bloem F H, Peng L, Pace N L, Crapo R O, Drews D, Clemmer T P (1990): “Automated sulfur hexafluoride washout functional residual capacity measurement system for any mode of mechanical ventilation as well as spontaneous respiration”, Crit. Care Med 18(1):84-91    Wahba RWM (1991): “Perioperative functional residual capacity”, Can J Anaesth 38(3):384-400    Fretschner R, Deusch H, Weitnauer A, Brunner JX (1993): “A simple method to estimate functional residual capacity in mechanically ventilated patients”, Intensive Care Med 19:372-376
However, a common problem with all the known methods for estimation of FRC are that they are difficult, complex and expensive to use. Moreover, they can e.g. not be used for continuous monitoring with repeated measurements, and they are often unreliable. Accordingly, FRC is a parameter which is very difficult to determine and to monitor, especially in patients with acute respiratory failure. It is also important to monitor FRC in other conditions, since a reduction of FRC frequently indicates an early stage of disease and may assist in the identification of a patient which will subsequently need respirator or ventilator treatment. However, ventilator treatment is very expensive and may require the resources of an intensive care unit. It is therefore important to identify such patients at an early stage, whereby preventive measures, such as CPAP-ventilation, could be taken in order to reduce the number of prospective ventilator/respirator users.
In a patient which requires a ventilator, restoration of the FRC is an important measure. FRC may be restored by application of an enhanced end expiratory pressure. However, today there is no clinically usable objective method for measuring FRC.
In many types of pulmonary (lung) diseases it is also important to follow (monitor) the development of FRC over time, for diagnostic and therapeutic reasons.
For non-ventilator patients, there is still a need for adequate and reliable measurement of FRC. Today, a body plethysmograph is used which is not easy to use, nor to move around.
Another physiological variable which is difficult to measure and monitor is Cardiac Output (CO). A current and most frequently used method is based on invasive application of a pulmonary catheter with thermal dilution technique. However, this method is complex, expensive, and difficult to use. Still further, the methodological procedure contains a considerable health risk for the patient. Thus, the method is currently only used in severe cases in intensive care, even though the clinical need for reliable CO measurements is considerably higher. In particular, there is frequently a need for continuous monitoring of CO, which is today not achievable.
A reliable CO measurement is also important for healthy persons and persons with spontaneous respiration, and not only for diagnostic purposes in medical attendance. For example, there is a need for CO measurement for individuals undergoing exercise and physical capacity tests or evaluations.